Generally tire wear leads to lowered draining performance and longer braking distance on wet road surfaces. Also, in the case of winter tires, wear can bring about a marked drop in grip performance on an icy or snow-covered road. Furthermore, excessive wear can cause water to seep into the tread belts. Also, with small vehicles, when the remaining groove depth of a tire is reduced to 1.6 mm, a rubber projection called a “slip sign” makes its appearance in the tire groove. To ensure running safety of a vehicle, tires should be replaced before the appearance of the slip sign, but there are, in fact, not a few drivers who are least interested in such maintenance practice.
Thus, there is demand for technologies for automatically detecting wear of tires to warn the driver of the state. Also, in the light of vehicular control, it is desired that control for greater safety be realized through the grasp of changes in tire characteristics resulting from wear.
In a conventional method for estimating wear of a tire, the absolute speed of a vehicle is calculated using GPS, an optical sensor, or the like, then the dynamic radius of the tire is calculated by comparing it with the rotational speed of the wheel, and the amount of tire wear is derived from the difference between the dynamic radius of the tire and the radius of the tire when it is new (refer to Patent Documents 1 and 3, for example).
However, even for a completely worn tire, the difference in rotational speed between the worn tire and the new tire is about 1% at most. It has therefore been difficult to achieve a constantly accurate estimation of tire wear from the difference in tire radius because of the necessity not only to measure with great accuracy but also to take into consideration various error factors in vehicular running, such as the inner-outer wheel errors in cornering, errors due to acceleration slips at braking or driving, and errors due to inclinations on the road surface.
Also, there have been other methods proposed for estimating tire wear, in which a transponder or an IC tag is embedded in the tire tread together with a receiver installed on the vehicle body in order to estimate tire wear using the loss of response as the transponder or the IC tag is broken or falls off when the tire is worn (refer to Patent Documents 3 to 5, for example), or a detector consisting of a magnetic material or a conductive rubber is embedded in the tire tread together with a sensor installed on the vehicle body in order to estimate tire wear by detecting the change in the signals detected by the sensor as the detector wears off as a result of tire wear (refer to Patent Documents 6 and 7, for example).
However, the methods using a transponder or IC tag or a magnetic material or conductive rubber embedded in the tire tread have not only the problem of adverse effects on the durability of the tire when the detector or sensor is exposed in the contact patch with the progress of wear but also the problem of lowered grip force of the tire when the rubber whose physical properties are different from those of the tread rubber is exposed on the surface of the tire tread.
Also, there has been a disclosure of technological thought, in which a strain sensor for detecting a strain of a tire in the circumferential direction is disposed to measure the state of tire wear from the waveform of strain outputted by the strain sensor. The fact of the matter, though, is that there has been no disclosure of the concrete structure thereof (refer to Patent Document 8, for example). Moreover, the invention disclosed in this document specifies a low-speed domain of 10 km/h or below, for instance, as preferred. As such, the measurement can only be made in nearly static conditions, and so it suffers from the drawback of inability to measure the state of wear during running at high speeds or on a rough road, for instance.
In this connection, the present applicant proposed a method for estimating the degree of tire wear using an index of deformation speed in the tire radial direction at the contact edge of the tire tread (WO2009008502). In this method, an acceleration sensor is installed in the inner liner region of a tire, and a radial acceleration of the tire is detected. Then a temporally-differentiated waveform thereof is calculated, and differential peak values, which are the magnitude of peaks appearing in the calculated differentiated waveform of radial acceleration, are calculated so as to use them as the index of deformation speed. And the degree of wear of the tire is estimated using the index of deformation speed and a predetermined relationship between the index of deformation speed and the degree of tire wear. Thus, the wear of a tire can be estimated with excellent accuracy irrespective of differences in wear mode.
Prior Art Document
Patent Document
    Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-278419    Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-164830    Patent Document 3: Japanese Unexamined Patent Application Publication No. 10-307981    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-205437    Patent Document 5: US2002/0116992A1    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2003-214808    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2005-28950    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2007-153034